1. Field of the Invention
This invention relates to a cord tensioner for a cord-guided covering for an architectural opening.
2. Description of the Relevant Art
Such a cord-guided covering can be a roller blind, a pleated blind, a venetian blind, or a combination of such blinds. And such a blind or combination of blinds can be mounted in an architectural opening such as e.g. a window or in a slanted window, such as a roof window.
Cord tensioners for cord-guided coverings have been conventionally used to keep the guide cords of the window coverings taut. One of the effects of the cord tensioners is that a movable rail such as a head, intermeditate or bottom rail, which slide along the cords, will be kept in place when they are not being operated. The movable rails are particularly kept from sliding down along the cords, under their own weight by the tension of the cords and the friction between the cords and the movable rails. In slanted windows, the taut cords prevent the fabrics and slats of window coverings from sagging downwardly away from the windows.
Cord tensioners traditionally are common type tension springs. Such tension spring type cord tensioners for guide cords of window coverings are described in U.S. Pat. No. 4,733,711 and U.S. Pat. No. 4,557,309. However, tension springs have the inherent problem that they can be over-stretched, particularly when movable rails of their window coverings are moved up or down abruptly, so as to stretch the guide cords. Such springs can then be over-tensioned before the movable rail starts to slide along the guide cords, since the springs will bear the sudden, though temporary, force. Also when in the life-time of a blind, the guide cords extend in length by wear and tear, and the cords have to be re-tensioned, generally the tension spring is pulled out further.
It has been known to protect a tension spring against over-tension by mounting a cord between the two spring ends, the cord having a length that is shorter than the maximum expanding length of the spring. A problem with this solution is that the cord must always too long for the spring in normal use, and the excess length can get easily snagged between the coils of the spring.
Another solution, described in EP 0,699,268, is to provide two looped cords that are slidably attached to opposite ends of a tension spring and two stops that are spaced apart from opposite ends of the tension spring to prevent the spring from being over-tensioned. A drawback of this solution is the need of providing extra space where the tension spring is placed. Also, since a single tension spring is generally used, the guide cords must be made as looped cords.